Agricultural Engineering · Energy, Structures & Environment · also Biomass Energy, Biochemical and Thermochemical Conversion, Liquid Biofuels
Turning crop residues, dung and oils into fuel: briquettes, gasifiers, biogas plants and liquid biofuels, and auditing farm energy.
In the exams: ICAR AIEEA-PG Unit V; ICAR AICE subject 42 Units 1–3 and 8; ICAR AICE subject 39 Unit 9; GATE AG biogas sizing (2 of 8 papers)
Bio-energy systems engineering is the design and operation of equipment that turns biomass (crop residues, wood, dung, oilseeds and sugar crops) into heat, power and fuels. Thermochemical routes use heat: combustion burns biomass completely, pyrolysis heats it without air to make char, oil and gas, and gasification burns part of it to make a combustible gas. Biochemical routes use microbes: anaerobic digestion makes biogas and yeast fermentation makes ethanol. Densification into briquettes and pellets, and the transesterification of oils into biodiesel, complete the toolkit.
Agriculture both produces large amounts of residue and consumes much energy. Rice mills burn husk for parboiling steam, tea and spice factories need drying heat, dairy farms produce manure that can be digested, and many villages still cook on biomass. An engineer who can characterise a residue, size a digester, choose a gasifier or check a biodiesel mass balance can cut fuel bills and pollution together, while remembering that residues also serve as fodder, bedding and soil organic matter, and that many small plants fail from poor maintenance rather than poor design.
Examiners test the same core worldwide. Indian postgraduate entrance papers (ICAR AIEEA-PG, GATE AG) ask short numericals on heating value, gasifier efficiency and biogas plant volume, and recall of digestion stages and plant types. PhD entrance and recruitment papers (ICAR AICE-JRF/SRF, ASRB NET) go deeper into reaction chemistry, kinetics and reactor design. Licensure boards such as the Philippine agricultural and biosystems engineering examination and the US PE Agricultural and Biological Engineering examination set applied design and energy-balance problems. Typical values in this manual are ranges from standard texts; feedstocks vary with species, climate and handling, and national standards differ, so measure or take design values from the standard in force.
Biomass stores solar energy captured by photosynthesis. For energy work the key cell-wall components are cellulose, hemicellulose and lignin, plus extractives and inorganic ash. Cellulose, a partly crystalline polymer of glucose, typically forms roughly 35–50% of the dry mass of wood and straw. Hemicellulose, a branched polymer of several sugars including the five-carbon sugars xylose and arabinose, typically forms roughly 20–35%. Lignin, an aromatic polymer that stiffens the wall, is commonly about 10–25% in straws and more in woods. Sugars and starch ferment easily; cellulose ferments only after pretreatment and hydrolysis; lignin hardly digests but carries much energy and binds briquettes.
Field residues (straw, stalks, cobs) are left after harvest; process residues (husk, bagasse, shells, press mud) arise at the mill. Quantities are estimated with a residue-to-product ratio, which varies with variety, harvest height and region, so local surveys beat textbook figures. Fodder, bedding, thatching and soil organic matter compete for residue, so an assessment reports gross generation, current uses and the surplus that can be collected sustainably.
Proximate analysis divides a fuel into four fractions by heating tests. Moisture is lost on drying at about 105 °C to constant mass. Volatile matter is the further loss when the dry sample is heated to about 900–950 °C for a few minutes in a covered crucible without air. Ash is the residue after burning in air; for biomass this is done at about 550–600 °C, lower than the roughly 750 °C used for coal, because biomass ash is rich in potassium, sodium, chlorine and carbonates that volatilise or decompose at higher temperature. Fixed carbon is 100 minus the other three. ASTM (for example E871, E872 and E1755) and ISO publish methods for biomass; state the method used.
Every result carries a basis. Dry-basis value = as-received value ÷ (1 − M); dry ash-free (daf) value = as-received value ÷ (1 − M − A), where M and A are the moisture and ash fractions. Fuel moisture is usually quoted on a wet basis (water ÷ total mass); some texts use a dry basis (water ÷ dry matter), and 50% wet basis equals 100% dry basis.
Biomass has high volatile matter, roughly 65–85% of dry mass, and low fixed carbon, roughly 10–25%, so it ignites easily and suits pyrolysis and gasification. Ash varies widely: clean wood often under 1–2%, straw several percent to above 10%, rice husk commonly 15–25%, mostly silica.
Ultimate analysis reports carbon, hydrogen, nitrogen, sulphur and oxygen, usually on a dry basis, with ash. C, H, N and S are measured in an automated analyser from the combustion gases; oxygen is found by difference, O = 100 − (C + H + N + S + ash). Forgetting ash overstates oxygen and understates calculated heating value.
Dry wood typically contains roughly 47–52% carbon, about 6% hydrogen and roughly 38–45% oxygen, with nitrogen and sulphur usually well under 1%. Compared with coal, biomass is richer in oxygen and poorer in carbon, so its heating value and its air requirement per kg are lower; its low sulphur is an environmental advantage. On a van Krevelen diagram (atomic H/C against O/C), torrefaction, carbonisation and coalification lower both ratios and raise heating value.
Minor elements matter for equipment: potassium and chlorine, high in straws, cause slagging, fouling and corrosion; silica dominates rice husk ash; nitrogen sets fuel-NOx and the C:N ratio for digestion. Physical properties (particle size, bulk density, which for loose straw is commonly well below 150 kg/m³, and flowability) often decide the technology more than chemistry does.
The heating (calorific) value is the heat released when unit mass of fuel burns completely and the products return to a reference temperature, usually 25 °C. The higher (gross) heating value, HHV, counts the latent heat of all product water condensed to liquid; the lower (net) heating value, LHV, assumes the water leaves as vapour. Most appliances vent vapour, so their efficiencies are usually quoted on LHV; laboratory reports often give HHV. Always state which.
Product water comes from fuel moisture and from hydrogen, each kilogram of which forms 9 kg of water. With latent heat at 25 °C of about 2.442 MJ/kg, on a dry basis LHV = HHV − 2.442 × 9H. For moisture fraction M (wet basis), per kg of wet fuel LHV_wet = HHV_dry(1 − M) − 2.442[9H(1 − M) + M]. For typical wood this reaches zero near 85–90% moisture, but stable burning fails long before, and most appliances need fuel far drier than 50%.
Typical dry HHVs: wood roughly 18–20 MJ/kg, many straws roughly 15–17, rice husk commonly 13–16 (high ash), charcoal roughly 28–33. Methane is about 55.5 MJ/kg HHV and 50 LHV. By volume, LHVs are about 35.8 MJ/Nm³ for methane, 12.6 for carbon monoxide and 10.8 for hydrogen.
The oxygen bomb calorimeter is the reference instrument. About one gram of pelleted sample burns in a steel bomb filled with oxygen at about 25–30 atmospheres, immersed in a known mass of water; a fuse wire ignites it and the water temperature rise is measured precisely.
Heat released = (mass of water + water equivalent of apparatus) × specific heat of water × temperature rise, corrected for fuse-wire heat, acid formation and heat exchange with the surroundings. The water equivalent, the mass of water that would absorb the same heat as the bomb, bucket and stirrer, is found by burning benzoic acid (about 26.45 MJ/kg). Because products cool inside a closed vessel and water condenses, the result is the gross heating value at constant volume; the net value is then calculated from hydrogen and moisture. Dry and grind samples, run replicates, recalibrate regularly and report the basis.
Without a calorimeter, heating value can be estimated from composition. Dulong's formula, developed for coal, is HHV (MJ/kg) ≈ 33.8C + 144(H − O/8) + 9.4S with mass fractions; H − O/8 is the 'available' hydrogen. It tends to under-predict for oxygen-rich biomass, so correlations fitted to broad data are preferred, notably Channiwala and Parikh (2002): HHV = 0.3491C + 1.1783H + 0.1005S − 0.1034O − 0.0151N − 0.0211A (MJ/kg, mass percent, dry basis), usually within a few percent of measurement. Proximate-analysis correlations exist but hold only for the fuels they were fitted to.
For gases, heating value is the volume-weighted sum of component values: producer gas with 20% CO, 18% H2 and 2% CH4 has LHV ≈ 0.20 × 12.63 + 0.18 × 10.78 + 0.02 × 35.8 ≈ 5.2 MJ/Nm³. For resource estimates, residue mass × heating value gives the gross energy: 1000 t of paddy yields roughly 200 t of husk (the share varies with variety), about 2600 GJ at 13 MJ/kg, of which appliances turn only part into useful heat.
Many residues are bulky, dusty and irregular. Loose straw, husk, sawdust and shells have bulk densities often between about 50 and 200 kg/m³, so they are costly to collect, transport and store, and they burn unevenly because they pack poorly and fall through grates. Densification compresses them into briquettes (typically 25–100 mm diameter logs or blocks) or pellets (typically 6–8 mm diameter cylinders). The unit density of a good briquette or pellet is commonly around 1000–1400 kg/m³, and the bulk density of pellets in a heap is typically about 600–700 kg/m³, several times that of the loose material.
Densification does not raise the heating value of the dry matter, and it cannot remove ash. What it does is raise bulk and energy density, make the fuel uniform in size and moisture, reduce dust, allow automatic feeding in stoves and boilers, and open markets for residues that would otherwise be burned in the field. The compaction ratio, density after ÷ density before, is a simple measure of the change, but a fair comparison uses the bulk density of stacked briquettes rather than the unit density of one briquette.
Particles stick together by several mechanisms. Solid bridges form when lignin, sugars or other natural components soften under heat and pressure, flow between particles, and set on cooling. Mechanical interlocking of fibres helps, as do attractive forces at close contact. Added binders such as starch, molasses, clay or cow dung create bridges in low-pressure processes.
Lignin is the key natural binder in high-pressure, binderless densification. It softens when heated, and moisture lowers its softening temperature, so both die temperature and feed moisture matter. For most binderless briquetting and pelleting, feed moisture of about 8–12% (wet basis) is recommended; the exact optimum depends on the material and machine. Too little moisture gives poor bonding; too much turns to steam in the hot die and cracks or bursts the product. Particle size must also suit the process: fines fill voids and bond well, but very fine material raises energy use, while coarse pieces break the briquette. Feedstocks are therefore usually dried, ground (commonly to a few millimetres) and sometimes conditioned with steam before densification.
Piston (ram) presses push material through a tapered die with a reciprocating ram driven by a flywheel and crank (mechanical) or by hydraulics. They make solid cylindrical briquettes, typically 40–100 mm in diameter, at high pressure; wear parts last reasonably long. Hydraulic presses work at lower pressure and give lower-density briquettes, but tolerate a wider range of moisture and particle size.
Screw extruders force material continuously through a heated die with a rotating screw. The screw shaft leaves a central hole along the briquette, which improves combustion, and the hot die partly carbonises the surface, improving weather resistance and handling. The price is rapid screw wear, requiring frequent hard-facing or replacement, and higher specific energy consumption.
Pellet mills use a ring die or a flat die with rollers that force material through many small holes. The die's compression ratio, the effective channel length divided by hole diameter, controls back-pressure: a higher ratio gives denser, more durable pellets but uses more energy and risks choking. Different feedstocks need different dies. Roller (briquette) presses and low-pressure manual presses with binders serve small users, including char-briquette makers who bind carbonised residue with starch or clay.
Densified fuel is judged by moisture, ash, unit and bulk density, mechanical durability, dimensions, heating value and, for boilers, ash melting behaviour and chlorine content. Mechanical durability is measured by tumbling a sample in a rotating box or drum for a set number of revolutions, sieving out fines and reporting the percentage retained; low durability means dust, losses and feeding problems. Shatter tests and compressive strength tests are also used for briquettes. International specifications for wood and non-woody pellets and briquettes are set in the ISO 17225 series of solid biofuel standards, with national standards in many countries.
Specific energy consumption, energy used per tonne of product, is the key operating figure; it is simply motor power divided by throughput (for example 30 kW at 0.5 t/h gives 60 kWh/t) plus the energy for drying and grinding, which may exceed that for pressing when the feed is wet. Economic viability depends on securing feedstock at stable prices, keeping wear costs down and finding buyers; many plants have failed on supply and marketing rather than technology.
Combustion is the rapid oxidation of fuel to release heat. A burning biomass particle passes through four stages: drying, as heat drives off moisture; devolatilisation (pyrolysis), as the dry solid decomposes into volatile gases and vapours and a porous char; flaming combustion of the volatiles in the gas phase around the particle; and char combustion, the slower glowing oxidation of the remaining carbon at the particle surface. Because biomass is mostly volatile matter, most of its heat is released in the flame, and good combustion design is largely about burning volatiles completely.
Complete combustion needs the 'three Ts' — high enough temperature, enough residence time and good turbulence to mix fuel gases with oxygen — plus sufficient oxygen. Too little air gives smoke, soot and carbon monoxide; too much air cools the flame and carries heat out of the stack. Designers therefore separate primary air, supplied under or through the fuel bed, from secondary air, injected above the bed into the volatile flames, and provide a hot, well-mixed combustion space.
Oxygen required per kg of fuel is (32/12)C + 8H + S − O kg, and stoichiometric air is that ÷ 0.232, the mass fraction of oxygen in air. For dry wood with C = 0.48, H = 0.06 and O = 0.42 this gives about 1.34 kg oxygen and 5.8 kg air per kg, roughly half the requirement of coal because biomass carries its own oxygen. Using 0.21 (a volume fraction) in a mass calculation is a common slip.
Appliances run with excess air: actual air = stoichiometric air × (1 + excess fraction). It is checked from dry flue-gas oxygen with excess air ≈ O2 ÷ (21 − O2); 6% O2 means about 40% excess.
Losses are mainly sensible heat in dry flue gas (rising with stack temperature and excess air), latent heat of water vapour, unburned CO and unburned carbon in ash. Thermal efficiency = useful heat ÷ fuel energy. For a dryer furnace, useful heat = air mass flow × specific heat × temperature rise; in a stove water-boiling test, it is the sensible heat of the water plus the latent heat of water evaporated. Improved stoves with enclosed, insulated chambers do much better than open fires, but figures depend strongly on test protocol.
Domestic cookstoves range from open fires to improved stoves with insulated combustion chambers, chimneys or forced draft; performance is assessed by standardised water-boiling and cooking tests measuring fuel use, time and emissions. Household smoke is a major health hazard, so emissions of particulate matter and CO matter as much as efficiency.
Grate furnaces burn chunky fuels on fixed, inclined, moving or travelling grates; they are robust and tolerate varied fuels. Husk-fired furnaces, including inclined-grate and cyclonic designs, supply clean hot air or flue gas for grain dryers and parboiling boilers. Fluidised-bed combustors suspend fuel in a hot bed of sand fluidised by air; excellent mixing gives good burnout at bed temperatures of about 800–900 °C, which keeps the bed below the ash-softening temperature and limits NOx. Bubbling beds suit medium plants; circulating beds suit large ones. Co-firing biomass with coal in existing power boilers uses established plant but is limited by fuel handling and ash behaviour.
Boilers produce steam for processing or power. Small biomass power stations generate electricity through a steam turbine; their efficiency is modest at small scale, so combined heat and power, which uses exhaust or extraction steam for process heat, is usually more sensible where there is a heat demand.
Ash causes most operational trouble in biomass combustion. Slagging is the fusion of ash into glassy lumps on grates and furnace walls; fouling is the build-up of deposits on cooler heat-transfer surfaces. Both are driven by potassium, which forms low-melting silicates, and by chlorine, which carries potassium into the gas as KCl that condenses on tubes and causes corrosion. Straw and many grasses are more troublesome than wood for this reason. Remedies include lower combustion temperature, fluidised beds, additives such as kaolin or lime, fuel blending, and washing or weathering straw to leach potassium and chlorine.
Rice husk ash is mostly silica. Burned at moderate temperature it stays largely amorphous and is a useful additive for cement and concrete; burned too hot or too long it crystallises, and crystalline silica dust is a lung hazard. Emissions to control in all biomass combustion are particulate matter (with cyclones, bag filters or electrostatic precipitators in larger plants), CO and unburned hydrocarbons (with good combustion), and NOx (mainly from fuel nitrogen). Open burning of crop residues in fields wastes energy and causes severe seasonal air pollution in several regions; collection for energy use is one of the remedies promoted by governments.
Pyrolysis is thermal decomposition of biomass in the absence, or near absence, of oxygen, and is the first step of both combustion and gasification. It yields char, a condensable liquid (bio-oil, tar and water) and non-condensable gas (CO, CO2, H2, CH4). Hemicellulose decomposes first, roughly 200–350 °C; cellulose over a narrower band around 300–400 °C, giving most of the vapours; lignin slowly over a very wide range, giving most of the char.
Products depend on temperature, heating rate and vapour residence time. Slow heating over hours to days maximises char (carbonisation). Fast heating to about 500 °C, vapour residence of one to two seconds and rapid quenching maximise liquid (fast pyrolysis). Higher temperature or longer residence cracks vapours to gas. Reviews report roughly 75% liquid, 12% char and 13% gas for fast pyrolysis and roughly 30% liquid, 35% char and 35% gas for slow carbonisation, varying with feed and reactor. Energy yield = mass yield × (product HHV ÷ feed HHV), so charcoal keeps more of the energy than its mass share suggests.
Torrefaction is mild pyrolysis at about 200–300 °C in an inert atmosphere for tens of minutes. It removes water and light volatiles, mainly from hemicellulose, leaving a brittle, water-repellent solid that grinds easily. Energy densification = energy yield ÷ mass yield: keeping 70% of mass and 85% of energy raises heating value per kg about 1.2 times.
Charcoal remains a major fuel in many countries, especially in urban Africa and South Asia. Traditional earth-mound and pit kilns burn part of the wood charge to supply heat, and let pyrolysis vapours escape as dense smoke; their charcoal yield is low and they waste wood and pollute. Improved kilns of brick or metal, and retorts that heat the charge indirectly and burn the vapours as fuel, give better and more consistent yields with far less smoke. Yield also depends on wood moisture (dry wood gives better yields), final temperature (higher temperature raises fixed carbon but lowers yield) and operator skill.
Residues such as husk, cotton stalk and coconut shell can be carbonised in drum or continuous kilns; the resulting char fines are usually bound with a starch or clay binder into char briquettes. The same carbonised product applied to soil is called biochar; claims for its effect on yield and soil carbon vary strongly with soil, climate and char properties, and should be judged from local trials.
Fast pyrolysis needs very rapid heat transfer to small, dry particles (commonly below a few millimetres and about 10% moisture), a controlled reaction temperature, rapid removal of char, and rapid quenching of vapours. Reactor types include bubbling and circulating fluidised beds (the most developed), ablative reactors that press particles against a hot surface, rotating-cone reactors, and auger reactors.
Bio-oil is a dark, dense liquid quite unlike petroleum. It is acidic (pH commonly about 2–3), contains roughly 15–30% water and roughly as much oxygen as the feed, has a heating value of about 16–19 MJ/kg, and tends to thicken and separate in storage. It does not mix with hydrocarbon fuels. It can be burned in adapted boilers and furnaces, used as a source of chemicals and smoke flavourings, or upgraded by hydrotreating or catalytic cracking, but it is not a drop-in engine fuel. The gas and char from fast pyrolysis are usually burned to heat the process.
Gasification converts solid biomass into a combustible gas by partial oxidation. Air, oxygen or steam is supplied at well below the amount needed for complete combustion; for air gasification the equivalence ratio (actual air–fuel ratio ÷ stoichiometric air–fuel ratio) is usually about 0.2–0.4. The heat from burning part of the fuel drives drying, pyrolysis and the endothermic reduction reactions that make the fuel gas.
In a fixed-bed gasifier four zones can be identified: drying (up to roughly 150–200 °C); pyrolysis (roughly 200–600 °C), producing char, tars and gases; oxidation (combustion), the hottest zone, often around 1000–1400 °C at the air inlet, where C + O2 → CO2 (about −394 kJ/mol) and partial oxidation C + ½O2 → CO (about −111 kJ/mol) release heat; and reduction, where hot CO2 and H2O react with glowing char. The main reduction reactions are the Boudouard reaction C + CO2 → 2CO (about +172 kJ/mol) and the water–gas reaction C + H2O → CO + H2 (about +131 kJ/mol), both endothermic, so the gas cools as it passes through the reduction zone, typically to around 700–900 °C. The water–gas shift CO + H2O ⇌ CO2 + H2 (about −41 kJ/mol) and methanation C + 2H2 → CH4 (about −75 kJ/mol) adjust the final composition.
Raising the equivalence ratio burns more fuel: temperature rises and tar falls, but the gas carries more CO2, H2O and nitrogen and its heating value drops. Cold gas efficiency usually peaks in the region of about 0.25–0.35.
In an updraft gasifier fuel is fed at the top and moves down, air enters through a grate at the bottom, and gas flows upward against the fuel. The oxidation zone sits just above the grate, followed upward by reduction, pyrolysis and drying zones. The gas leaves at the top at a low temperature after passing through the pyrolysis and drying zones, so it carries the pyrolysis tars and moisture with it.
Advantages: high thermal efficiency, because the hot gas gives up its heat to the incoming fuel; simple construction; tolerance of wetter and more varied fuel; and good char burnout because gas passes through the whole bed. Disadvantage: a very tarry gas, commonly tens of grams of tar per normal cubic metre, which is acceptable when the gas is burned directly, close-coupled, in a furnace or kiln, but is unsuitable for engines without heavy cleaning. Updraft units therefore serve thermal applications.
In a downdraft gasifier fuel and gas both move downward. Air is injected through nozzles at the oxidation zone, often at a constriction called the throat (the Imbert design), and the gas leaves below the grate. Pyrolysis vapours must pass through the hottest zone and then through the bed of glowing char, where most tar is cracked or burned. With good design and operation, tar in the raw gas is far lower than in updraft units, which is why downdraft gasifiers are preferred for engines. The trade-offs are: the gas leaves hot, so sensible heat is lost unless recovered; the fuel must be fairly dry (commonly below about 20% moisture), of suitable size and not too high in ash; and scale is limited, because it is hard to keep the air penetrating a wide throat, so throated downdrafts are mostly below about 1 MW thermal.
Open-top, throatless (stratified) downdraft reactors use a wide cylinder and are better suited to fine, high-ash fuels such as rice husk, which bridge and choke in a narrow throat. Cross-draft gasifiers, with air entering from one side and gas leaving the other, respond quickly to load but need low-tar fuel such as charcoal. Fluidised-bed gasifiers (bubbling or circulating) suit larger capacities and varied fuels, and entrained-flow gasifiers suit finely ground or liquid feeds at large scale.
Cold gas efficiency is the chemical energy in the cooled gas divided by the energy in the fuel: (gas flow × gas LHV) ÷ (fuel rate × fuel LHV). Well-run small wood gasifiers commonly achieve about 60–80%. Hot gas efficiency adds the sensible heat of the gas and applies when hot gas is burned directly. Gas yield for air-blown wood gasification is commonly about 2–2.5 Nm³ per kg of fuel; it can be found from a nitrogen balance, since the nitrogen in the air leaves unchanged in the gas: gas volume = air volume × 0.79 ÷ N2 fraction in gas.
The key sizing parameter for fixed-bed reactors is the specific gasification rate (hearth load), the fuel consumed per hour per square metre of reactor or throat cross-section. For a chosen rate, the required fuel consumption fixes the diameter. Turndown, the ratio of maximum to minimum stable output, is limited in throated designs because low flows let tar pass uncracked.
Gas for engines must be cooled and cleaned. A typical train has a cyclone for coarse particles, a cooler or water scrubber, and filters of fabric, sawdust or other media for fine dust and tar mist. Wet scrubbing produces effluent containing tar and phenolic compounds that is toxic and must be recirculated and treated, never simply discharged. Carbon monoxide in producer gas makes leaks dangerous: sheds must be ventilated, start-up gas flared, and CO detectors used.
Air-blown wood gas is typically about 15–22% CO, 12–20% H2, 1–3% CH4, 8–12% CO2 and roughly half nitrogen, with an LHV of about 4–6 MJ/Nm³. It needs only about one volume of air per volume of gas: for 20% CO, 18% H2 and 2% CH4 the oxygen needed is 0.10 + 0.09 + 0.04 = 0.23 m³, or about 1.1 m³ of air. The stoichiometric mixture therefore holds about 2.5 MJ/m³, against roughly 3.5–3.8 MJ/m³ for a petrol–air mixture; since cylinders fill with mixture, engines on producer gas are derated unless compression ratio or boost is raised. CO and CH4 resist knock, allowing higher compression ratios, and hydrogen raises flame speed. The gas must be cooled, to raise its density and condense tar and water before the engine, and cleaned of dust and tar; limits of tens to about 100 mg/Nm³ of tar are commonly quoted, but engine makers' requirements differ.
A compression-ignition engine cannot run on producer gas alone because the gas–air mixture does not auto-ignite reliably at diesel compression temperatures. In dual-fuel operation gas is mixed with intake air and a pilot diesel injection ignites the charge. The engine needs little modification, starts on diesel and can switch back at any time. Diesel replacement = (diesel alone − pilot diesel) ÷ diesel alone at the same output; it commonly reaches about 60–80% at moderate to high load with clean gas, less at light load where the minimum pilot is a larger share.
To remove diesel entirely, the engine is converted to spark ignition, usually with a lower compression ratio, or a gas engine is used; this is the norm for gasifier power plants above a few tens of kilowatts, and demands especially good gas cleaning. Small gasifier–engine sets commonly achieve about 15–25% overall biomass-to-electricity efficiency and about 1–1.5 kg of dry woody biomass per kWh. Biomass use = electrical power ÷ (efficiency × LHV): 20 kW at 20% with 15 MJ/kg fuel needs 20 × 3.6 ÷ (0.2 × 15) = 24 kg/h. Most plant failures come from gas quality and fuel handling, so logbooks of filter cleaning, condensate and oil condition matter.
Anaerobic digestion is the breakdown of organic matter by a consortium of microorganisms in the absence of oxygen, producing biogas (mainly methane and carbon dioxide) and a stabilised residue called digestate. It proceeds in four linked stages. In hydrolysis, extracellular enzymes break carbohydrates, proteins and fats into sugars, amino acids and long-chain fatty acids; for fibrous feeds such as straw, hydrolysis is the rate-limiting step. In acidogenesis, fermentative bacteria convert these products into volatile fatty acids (acetic, propionic, butyric), alcohols, hydrogen and CO2. In acetogenesis, acetogenic bacteria oxidise propionate, butyrate and alcohols to acetate, hydrogen and CO2. In methanogenesis, methanogenic archaea form methane.
About 70% of the methane in typical digesters comes from acetate, split by acetoclastic methanogens (CH3COOH → CH4 + CO2); the rest comes from hydrogenotrophic methanogens reducing CO2 with hydrogen (CO2 + 4H2 → CH4 + 2H2O). Acetogenesis of propionate and butyrate is thermodynamically possible only when hydrogen is removed as fast as it forms, so hydrogen-consuming methanogens and acetogens live in close partnership (syntrophy, or interspecies hydrogen transfer). When methanogens are stressed, hydrogen and then propionate accumulate, an early warning of trouble.
Because acid-forming bacteria grow much faster than methanogens, the most common failure is souring: a sudden load of easily degradable feed produces acids faster than methanogens can consume them, pH falls and methane production collapses.
Temperature. Digestion is classed as psychrophilic (below about 20 °C), mesophilic (about 30–40 °C, optimum near 35–37 °C) and thermophilic (about 50–60 °C, often 55 °C). Most farm and household digesters are unheated and mesophilic in warm climates; their gas output falls sharply in winter. Thermophilic digestion is faster and kills more pathogens but is more sensitive and needs heat. Sudden temperature changes upset the process more than steady lower temperatures.
pH and alkalinity. Methanogens work best near neutral pH, about 6.8–7.2, and are inhibited below about 6.5. Bicarbonate alkalinity buffers the process; the ratio of volatile fatty acids to alkalinity is a practical monitoring tool in larger plants.
Nutrients and C:N ratio. A carbon-to-nitrogen ratio of about 20:1 to 30:1 is recommended. Low ratios (for example poultry litter) release ammonia that inhibits methanogens, particularly at high pH and temperature; high ratios (straw alone) limit growth. Co-digestion of manure with crop residues or food waste balances the ratio.
Solids content, mixing and toxicity. Conventional wet digesters work with slurry of roughly 6–10% total solids; fresh cattle dung, commonly around 16–20% solids, is diluted with water, often 1:1 by mass, to reach this range. Mixing prevents scum and settled layers. Inhibitors include ammonia, sulphide, heavy metals, antibiotics and disinfectants in manure or wash water, and excess salts.
Biogas is typically 50–70% methane and 30–50% CO2, with traces of H2S, ammonia and water vapour; at 60% methane its LHV is about 21 MJ/m³. H2S is toxic and corrosive and is removed before engines and upgrading.
The Buswell equation gives the products of complete digestion of CnHaOb: CnHaOb + (n − a/4 − b/2)H2O → (n/2 + a/8 − b/4)CH4 + (n/2 − a/8 + b/4)CO2. Glucose gives 3CH4 + 3CO2 (50% methane, about 0.37 m³ CH4 per kg at standard conditions); a cellulose unit C6H10O5 takes up one water and gives about 0.41 m³/kg; fats give methane-richer gas. For wastewater, each kg of COD converted gives 0.35 m³ CH4 at 0 °C and 1 atm, because oxidising one mole of methane (22.4 L) needs 64 g of oxygen. Real yields fall short of theory because lignin shields cellulose, some substrate builds cells and not all degrades in the retention time; biochemical methane potential tests are the reliable design basis.
Digestate keeps the feed's N, P and K, much of the nitrogen as ammonium, and is a good fertiliser when stored and applied correctly. Mesophilic digestion reduces but does not eliminate pathogens.
The floating-drum plant, standardised in India by the Khadi and Village Industries Commission (KVIC) and widely known by that name, has a cylindrical underground digester of brick or concrete, often divided by a partition wall so that fresh slurry cannot short-circuit from inlet to outlet. A gas holder, traditionally a mild-steel drum, floats on the slurry or in a separate water jacket and rises and falls with the stored gas, guided by a central pipe and frame. The gas pressure equals the drum's weight divided by its area and stays nearly constant, commonly around 10 cm of water column; weights can be added to raise it. The height of the drum shows at a glance how much gas is stored.
Advantages are constant pressure, easy operation and visible storage. Disadvantages are the cost of the steel drum, corrosion that requires regular painting and eventual replacement, and the need to keep the drum moving freely. Drums of fibre-reinforced plastic or ferrocement are used to reduce corrosion.
In a fixed-dome plant the digester and gas store form one closed, usually underground, masonry or concrete structure with a dome roof. As gas accumulates it pushes slurry down in the digester and up into an outlet (displacement) chamber; the difference in slurry levels is the gas pressure, which therefore rises as gas is stored and falls as it is used, commonly reaching well above that of a floating drum. When gas is drawn, slurry flows back. Digested slurry overflows from the outlet chamber.
The Chinese fixed-dome design spread this principle widely. In India the Janata design was followed in the 1980s by the Deenbandhu design, developed by a voluntary agency (AFPRO), whose geometry made of spherical segments reduced the masonry surface and cost for a given volume; Deenbandhu-type plants became a mainstay of the national programme. Nepal's national programme has used a related fixed-dome design.
Fixed-dome plants are cheaper per unit volume, have no moving or corroding parts, save surface space and, being underground, stay warmer in cold seasons. Their weaknesses are variable gas pressure, which affects burner performance, and the need for skilled masons: the dome must be gas-tight, and cracks from poor workmanship or settlement are the most common cause of failure and are difficult to repair.
Bag or tubular (plug-flow) digesters use a long flexible plastic or rubber tube laid in a trench; they are cheap and quick to install, and suit warm climates, but are vulnerable to damage and sunlight degradation. Plug-flow digesters in concrete suit thicker manures (roughly 11–13% solids), which move through as a plug without mixing. Covered anaerobic lagoons capture methane from large liquid-manure lagoons in warm climates. Continuously stirred tank reactors (CSTRs), heated and mixed, are the standard for farm and co-digestion plants in Europe and North America and suit slurries of about 3–10% solids. High-rate reactors such as the upflow anaerobic sludge blanket (UASB) keep a granular sludge bed in the reactor while wastewater flows through, allowing hydraulic retention times of hours; they suit soluble effluents such as distillery, starch or dairy wastewater, not solid-rich manure. Dry (solid-state) digesters handle stackable feeds above roughly 20% solids.
A household plant has a mixing tank, inlet pipe, digester, gas holder or dome, outlet or displacement chamber, gas valve, pipeline and burners or lamps. Biogas is saturated with water vapour that condenses in the pipe, so the line must slope continuously to a water trap at its lowest point. Site the plant near the cattle shed and kitchen, away from drinking-water wells, on firm, sunny ground.
Biogas serves cooking, mantle lamps, dual-fuel or spark-ignition engines and, after upgrading (removing CO2, H2S and water by water scrubbing, pressure-swing adsorption, membranes or chemical absorption), compressed vehicle fuel. Never enter a digester or pit without ventilation: biogas can asphyxiate and H2S is toxic.
Two criteria set digester volume. The hydraulic retention time (HRT) is the average time slurry spends in the digester: HRT = digester volume ÷ daily slurry volume. So the digester volume is Vd = daily slurry volume × HRT. HRT must be long enough for slow-growing methanogens not to be washed out and for enough of the feed to degrade. For unheated household plants, national programmes specify HRTs of the order of several weeks, longer for cold regions and hill areas than for warm plains, because degradation slows with temperature; heated mesophilic farm CSTRs commonly run at shorter HRTs.
The organic loading rate (OLR) is the mass of volatile solids (or COD) fed per cubic metre of digester per day: Vd = daily VS load ÷ OLR. Overloading causes souring. A designer computes both volumes and adopts the larger, then adds a margin for the gas space where it is part of the same structure.
Slurry volume must include the dilution water. With fresh dung mixed 1:1 by mass with water and a slurry density taken as about 1000 kg/m³, 100 kg of dung per day gives about 0.2 m³ of slurry per day; at 40 days HRT that needs an 8 m³ digester. The water needed to dilute feed to a target solids content follows from a solids balance: TS_feed × 1 = TS_target × (1 + W). Diluting 18% solids dung to 8% needs 1.25 kg water per kg of dung.
Gas yield can be expressed per kg of fresh feed, of total solids (TS) or of volatile solids (VS). For fresh cattle dung, values of a few hundredths of a cubic metre per kg (about 0.036–0.04 m³/kg are commonly used in Indian design practice) reflect its high water content; diet, climate and plant change the figure. On a solids basis, gas = fresh feed × TS fraction × VS/TS × yield per kg VS added: 200 kg/day at 18% TS, VS 80% of TS and 0.25 m³/kg VS gives 7.2 m³/day.
A first-order model of batch digestion gives B(t) = B0[1 − exp(−kt)], with B0 the ultimate methane yield and k the rate constant. In an ideal completely mixed digester the fraction of B0 achieved at a given HRT is k·HRT ÷ (1 + k·HRT), lower than the batch value because some feed leaves early. These relations show the trade-off between digester size and yield. Working backward, animals needed = gas demand ÷ (dung per animal × gas yield); dung output varies with breed, feeding and how much is collected, so survey it. Overestimating available dung is the commonest sizing error.
Household demand combines a per-person cooking allowance (around 0.3–0.4 m³ per person per day appears in design manuals, but habits differ) with burner and lamp rates multiplied by hours of use; engines are rated per kWh. Storage bridges steady production and peaked use; where cooking falls in two or three meals, a holder of roughly half the daily production is common, but the real fraction comes from the use pattern.
For a floating drum, storage = (π D²/4) × effective rise, so D = √(4V ÷ πH); storing 0.85 m³ with 0.8 m rise needs D ≈ 1.16 m. For a fixed dome, storage is the dome volume between the lowest and highest slurry levels, and maximum pressure is set by the height between outlet overflow and the lowest digester level. Energy available = volume × methane fraction × 35.8 MJ/m³: 2 m³ at 60% holds about 43 MJ, or about 3 kWh of electricity at 25% generator efficiency, which is why household plants are cooking plants and electricity needs larger farm or community plants. In examinations all design values are given; in practice use local data or the national standard.
Ethanol is made by fermenting sugars, usually with the yeast Saccharomyces cerevisiae. Sugar feedstocks (cane juice, molasses, beet, sweet sorghum) ferment directly. Starch feedstocks (maize, wheat, cassava, broken rice) are cooked and hydrolysed with amylases to glucose. Lignocellulosic feedstocks need pretreatment and cellulases (second-generation ethanol).
Fermentation: C6H12O6 → 2C2H5OH + 2CO2, a theoretical 0.511 kg ethanol per kg hexose, with about 0.96 kg CO2 per kg ethanol. Sucrose takes up water on hydrolysis (C12H22O11 + H2O → 4C2H5OH + 4CO2), giving 0.538 kg/kg; starch gains 11% in mass on hydrolysis, giving 0.568 kg/kg in theory. Practical efficiencies are lower because yeast diverts sugar to growth and by-products such as glycerol. At about 0.789 kg/L, theoretical yield from glucose is about 0.65 L/kg.
Fermentation runs anaerobically at roughly pH 4–5 and 30–35 °C, and stops when ethanol reaches the yeast's tolerance, typically about 8–12% by volume in conventional batches. Bacterial contamination and cooling are the main operating concerns in warm climates.
The fermented mash (beer or wash) is distilled. Ethanol and water form an azeotrope at about 95.6% ethanol by mass, so ordinary distillation stops near that strength (rectified spirit). Fuel ethanol for blending must be nearly anhydrous, because water in a petrol blend causes phase separation; the remaining water is removed by molecular sieve adsorption (the dominant modern method) or by azeotropic distillation with an entrainer.
Distillation is energy-intensive, and its fuel is often bagasse or other biomass in sugar-mill distilleries. The residue after distillation is large in volume. From molasses it is spent wash (vinasse), dark and acidic with very high BOD and COD and high potassium; it is commonly treated by anaerobic digestion to biogas, then concentrated and incinerated, composted with press mud, or used on land where regulations allow. From grain it is stillage, which is dried to distillers' dried grains with solubles (DDGS), a protein-rich feed. Fermentation CO2 is high-purity and can be captured.
Lignocellulose resists conversion because cellulose microfibrils are embedded in hemicellulose and lignin. Pretreatment (steam explosion, dilute acid, hot water, alkali or ammonia-based) opens the structure, and cellulases release glucose. Cellulases are inhibited by cellobiose and glucose, so simultaneous saccharification and fermentation (SSF), where yeast consumes glucose as it forms, reduces inhibition at the cost of a compromise temperature between enzyme (about 45–50 °C) and yeast optima. Hemicellulose yields pentoses, mainly xylose, which native S. cerevisiae does not ferment, so engineered strains or other organisms are needed; lignin is burned for process energy. Enzyme cost, pretreatment and bulky feed have kept cellulosic ethanol small beside first-generation plants.
As a fuel, ethanol has a high octane rating, allowing higher compression ratios, but about 26.8 MJ/kg or 21 MJ/L, roughly two-thirds of petrol per litre, so consumption rises with blend level. It is hygroscopic, and blends can phase-separate if water enters. Its low cetane number rules out ordinary diesel use. Blend mandates such as E5–E20 and approved feedstocks differ by country and change often; life-cycle benefits depend on feedstock, land use, process energy and co-products.
Vegetable oils and animal fats are mainly triglycerides, esters of glycerol with three long-chain fatty acids. Straight vegetable oil can fuel some diesel engines, but its viscosity is roughly ten or more times that of diesel, causing poor atomisation, injector coking, piston-ring sticking and lubricating-oil dilution in most modern engines. Transesterification converts each triglyceride into three smaller molecules, fatty acid alkyl esters, with viscosity close to diesel. The product is biodiesel, usually fatty acid methyl ester (FAME).
Feedstocks include edible oils (rapeseed, soybean, palm, sunflower), non-edible oils suited to marginal land in some regions (jatropha, pongamia or karanja, mahua and others), used cooking oil and animal fats. Feedstock cost is usually the largest share of biodiesel cost, and quality, especially free fatty acid (FFA) and water content, decides the process route. Renewable diesel made by hydrotreating oils (hydrotreated vegetable oil) is a different, hydrocarbon product and is not transesterified.
Transesterification: triglyceride + 3 CH3OH ⇌ 3 methyl esters + glycerol, in three reversible steps through di- and monoglycerides. Excess methanol drives it forward; about 6:1 methanol-to-oil molar ratio (double the stoichiometric 3:1) is typical with an alkaline catalyst (NaOH, KOH or sodium methoxide) at roughly 0.5–1% of oil mass, near 60 °C (methanol boils at about 64.7 °C), for about an hour with good mixing.
With triolein (885.4 g/mol) and methyl oleate (296.5 g/mol), 1000 kg of oil (1.129 kmol) reacts with 108.6 kg of methanol to give 1004.6 kg of ester and 104.0 kg of glycerol: biodiesel mass about equals oil mass, and glycerol is about 10% of it. At 6:1 about 217 kg of methanol is charged and the excess recovered.
Free fatty acids react with alkali to form soap (RCOOH + NaOH → RCOONa + H2O), consuming catalyst, emulsifying the glycerol layer, and releasing water that hydrolyses esters. Acid value (mg KOH/g) measures FFA; as oleic acid, FFA% ≈ acid value ÷ 2. Oils above roughly 1–2% FFA are commonly given acid-catalysed esterification first, and the oil must be dry.
A batch process runs: pretreatment (filtering, drying, FFA reduction); reaction; settling or centrifuging, where the dense glycerol phase (about 1.26 kg/L) separates below the esters (about 0.88 kg/L); methanol recovery; washing with water or dry adsorbents to remove catalyst, soap, glycerol and methanol; and drying. Crude glycerol can be refined, burned or co-digested.
Standards such as ASTM D6751 and EN 14214 limit ester content, free and total glycerol, methanol, water, acid value, oxidation stability, viscosity, flash point, sulphur and cold-flow properties. Unconverted glycerides crystallise and plug filters.
Compared with diesel, biodiesel has higher cetane number, much better lubricity, negligible sulphur and about 11% oxygen; its heating value is about 10% lower per kg (roughly 37–40 against 42–45 MJ/kg). It cuts particulate, CO and hydrocarbon emissions but may raise NOx slightly, oxidises more readily, and has higher cloud and cold-filter plugging points, especially from saturated feeds such as palm oil or tallow. Blends such as B5, B7 or B20 are used subject to engine makers' approvals. Methanol is toxic and flammable, so small producers need safe storage and ventilation.
Energy auditing in agriculture measures how much energy of each kind goes into a product, where, and how efficiently; it underpins decisions on mechanisation, irrigation, fertiliser and processing, and feeds life-cycle assessment. Direct energy is used on site: human labour, animal draught, diesel, electricity. Indirect energy was spent making inputs: fertilisers, pesticides, seed, and machinery (apportioned over machine life). Renewable sources include labour, animal power, seed, manure and residues; non-renewable sources include fossil fuels, fertilisers, chemicals and machinery.
Inputs are converted with energy equivalents such as MJ per litre of diesel, per hour of labour and per kg of nutrient. These coefficients differ between sources, countries and years (nitrogen coefficients have fallen as ammonia plants improved), so every study must state them. Indian studies commonly use the ICAR manual compiled by Mittal and Dhawan and later compilations.
The main indicators are: energy ratio (energy use efficiency) = energy output ÷ energy input; energy productivity = main-product yield ÷ energy input (kg/MJ); specific energy = energy input ÷ yield (MJ/kg), the reciprocal of energy productivity; and net energy = output − input (MJ/ha). Output energy is the yield of main product and, if counted, by-product (straw) multiplied by their energy equivalents. Whether straw is counted as output changes the energy ratio greatly, so the choice must be stated.
A typical calculation lists each input per hectare, multiplies by its coefficient, sums to total input, and computes shares by source. In irrigated cereal systems, nitrogen fertiliser, diesel and electricity for pumping usually dominate; in rainfed systems labour and draught may dominate. For example, a crop with 15,000 MJ/ha input and 90,000 MJ/ha output has an energy ratio of 6.0 and net energy of 75,000 MJ/ha; if grain yield is 5000 kg/ha, energy productivity is 0.33 kg/MJ and specific energy 3 MJ/kg.
Energy ratios describe physical efficiency only. A system can have a high energy ratio and low profit, or the reverse, and the ratio says nothing about nutrition or environmental effects other than energy. It is most useful for comparing systems with the same boundary: tillage methods, irrigation methods, organic versus conventional nutrient supply, or mechanisation levels.
A walk-through audit collects fuel, electricity and biomass records and production data, lists the main energy users and computes specific energy consumption (energy per tonne of output) for benchmarking. A detailed audit measures motor loading and power factor, pump flow, head and efficiency, boiler and furnace efficiency by flue-gas analysis, steam and condensate losses, dryer air flows and lighting, then costs and ranks each saving by payback.
Common findings: irrigation pumps mismatched to well and pipeline, running at low efficiency; tractors oversized or poorly ballasted (fuel per hectare = fuel per hour ÷ field capacity); husk boilers and dryer furnaces with far more excess air than needed and bare steam lines in rice mills; and poorly insulated cold stores. Fixing losses is usually cheaper than adding supply.
Life-cycle assessment extends the audit to the whole chain. A biofuel's net energy ratio (fuel energy ÷ fossil energy used) depends strongly on the boundary and on how co-products such as oilcake, distillers' grains, glycerol or bagasse are credited, which is why published figures vary and remain debated.
Learn to keep bases straight (as-received, dry, dry ash-free; wet vs dry moisture) and to convert units between MJ, kWh and kW. Practise the standard calculations until they are automatic: dry and daf conversions, HHV to LHV, Dulong and unified correlations, stoichiometric air, equivalence ratio, cold gas efficiency, digester volume from HRT and loading, gas yield from volatile solids, Buswell yields, ethanol yield from sugar and starch, and the biodiesel mass balance. Visit a working biogas plant, a briquetting unit and a rice mill boiler if you can; seeing the scum layer, the drum guide or a choked throat makes the theory stick.
A field engineer five years in sizes household and farm digesters, supervises masons, commissions gasifiers for dryers and small power, and troubleshoots. Typical decisions: whether the dung actually collected supports the plant size a family wants; whether a husk furnace's flue oxygen shows excess air to cut; whether a gasifier's tar problem is fuel moisture, a burnt throat or poor filter maintenance; whether a digester's poor gas is overloading, cold weather or a cracked dome. Keep logbooks of fuel, gas and hours; most plant failures show in the numbers before they show on site.
A senior engineer with twenty-five years designs programmes rather than single plants: residue supply chains for briquetting and power plants, district biogas programmes with training and after-sales service, energy audits of rice and sugar mills, and the choice between combustion, gasification and digestion for a given feed and demand. The recurring lessons are that feedstock logistics and operator skill decide success, that residue has competing uses, and that subsidies should pay for performance rather than installation. Such a person also reviews designs against national standards and trains inspectors.
A researcher with long field experience works on the hard problems: tar formation and cracking, ash chemistry and agglomeration, kinetics of lignocellulose digestion and co-digestion, pretreatment and enzyme economics for cellulosic ethanol, catalysts for high-FFA oils, and honest life-cycle accounting of bioenergy with transparent boundaries and allocation. Good research here couples laboratory measurement (thermogravimetry, biochemical methane potential tests, gas chromatography) with long-duration field monitoring, and reports failures as well as successes so programmes can learn. Such a person also advises regulators on test protocols, standards and sustainability criteria for bioenergy schemes.
This manual gives principles, standard equations and typical ranges, not a design code. It does not reproduce the dimension tables of national standard biogas plant designs (KVIC, Deenbandhu, Janata or other countries' models), because those tables vary by edition and country; use the current standard drawings. It does not give country-specific energy equivalents, subsidy rules, blending mandates or emission limits, which change often. Gasifier sizing is covered through specific gasification rate and efficiency only; detailed throat and nozzle design rules, equilibrium modelling, and kinetic modelling of gasification are left to specialist texts. Algal biofuels, hydrothermal liquefaction, biohydrogen, microbial fuel cells, hydrotreated renewable diesel and power-plant steam-cycle design are mentioned at most in passing. Typical values for residue ratios, dung output and gas yields are illustrative ranges and should be replaced with local measurements.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Every numerical answer was computed by script; design values given in questions (gas yields, energy equivalents, retention times) are illustrative and vary by country and source.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.